1. Field of the Invention
The present invention relates to a connector suitable for injecting a dielectric enhancement fluid into the interstitial void volume of an electrical cable segment. More particularly, the invention relates to a high-pressure terminal connector and a high-pressure splice connector adapted for injecting the fluid at an elevated pressure and then confining the fluid within the void volume at a residual pressure, wherein pushback of the insulation jacket of the cable is essentially eliminated.
2. Description of the Related Art
Extensive networks of underground electrical cables are in place in many parts of the industrialized world. Such underground distribution offers great advantage over conventional overhead lines in that it is not subject to wind, ice or lightning damage. It is therefore viewed as a reliable means for delivering electrical power without obstructing the surrounding landscape, the latter feature being particularly appreciated in suburban and urban settings. Unfortunately, these cables, particularly those installed prior to 1985, which generally comprise a stranded conductor surrounded by a semi-conducting shield, a polymeric insulation jacket, and an insulation shield, often suffer premature breakdown and do not attain their originally anticipated longevity of 30 to 40 years. Their dielectric breakdown is generally attributed to so-called “treeing” phenomena (i.e., formation of microscopic voids or branching channels within the insulation material, from which the descriptive terminology derives), which lead to a progressive degradation of the cable's insulation. Since replacing a failed section of underground cable can be a very costly and involved procedure, there is a strong motivation on the part of the electrical utility industry to extend the useful life of existing underground cables in a cost-effective manner.
Many early efforts focused on rejuvenating in-service cables by either simply drying the insulation or introducing a tree retardant liquid into the void space (interstitial void volume) associated with the stranded conductor geometry after such a drying step (e.g., U.S. Pat. Nos. 4,545,133 and 4,372,988). The liquid was believed to diffuse out of the cable's interior and into the insulation, where it filled the microscopic trees and thereby augmented the service life of the cable.
An improvement over the above methods was proposed by Vincent et al. in U.S. Pat. No. 4,766,011, wherein the tree retardant liquid was selected from a particular class of aromatic alkoxysilanes which polymerized within the cable's interior as well as within the water tree voids in the insulation and therefore did not permeate rapidly out of the cable. This method and variations thereof employing certain rapidly diffusing components (see U.S. Pat. Nos. 5,372,840 and 5,372,841) have enjoyed commercial success over the last decade or so, but they still have some practical limitations when reclaiming underground residential distribution (URD) cables, which have a relatively small diameter, and therefore present insufficient interstitial volume relative to the amount of retardant required for optimum dielectric performance. Thus, although not explicitly required by the above mentioned disclosures, in-the-field reclamation of URD cables employing such silane-based compositions typically leaves a liquid reservoir connected to the cable for a 60 to 90 day “soak period” to allow additional retardant liquid to penetrate the cable insulation and thereby restore the dielectric properties. As a result, it is generally necessary to have a crew visit the site at least three times: first to begin the injection, which involves a vacuum at one end and a slightly pressurized feed reservoir on the other end; second to remove the vacuum bottle a few days later after the fluid has traversed the length of the cable; and finally to remove the reservoir after the soak period is complete. These repetitive trips are costly in terms of human resource. More importantly, each exposure of workers to energized equipment presents additional risk of serious injury or fatality and it would be beneficial to minimize such interactions. In view of the above limitations, a circuit owner might find it economically equivalent, or even advantageous, to completely replace a cable once it had deteriorated rather than avail himself of the above restorative methods.
In all of the above-recited methods for treating in-service cables, the tree retardant liquid is injected into the cable under a pressure sufficient to facilitate filling the interstitial void volume. And, although pressures as high as 400 psig have been employed to this end (e.g., Transmission & Distribution World, Jul. 1, 1999), the pressure is always discontinued after the cable is filled. At most, a residual pressure of up to about 30 psig is applied to a liquid reservoir after injection, as required for the soak period in the case of URD cable reclamation. Further, while higher pressures have been used to inject power cables, this prior use is solely to accelerate the cable segment filling time, especially for very long lengths as are encountered with submarine cables (e.g., the above Transmission & Distribution World article) or new cables injected with strand-blocking material (i.e., not a tree retardant or dielectric enhancing fluid) on-the-reel as contemplated by U.S. Pat. Nos. 4,845,309, 4,961,961, and 4,978,694.
Moreover, even when higher pressures were maintained in an experimental determination of possible detrimental effects of excessive pressure, a maximum pressure of 117 psig was maintained for only two hours. More to the point, in this experimental procedure the pressure was maintained for this brief period by an external pressure reservoir. (Entergy Metro Case Study: Post-Treatment Lessons, Glen Bertini, ICC April, 1997 Meeting, Scottsdale, Ariz.).
In the above methods, the liquid tree retardant was injected into the cable interior using special fittings comprising an injection port for the introduction of the tree retardant liquid and a means for sealing the device to the cable so that fluid would not leak out during injection. At relatively low injection pressures (e.g., less than about 30 psig), a small window could simply be cut into the cable insulation and a housing having an injection port clamped around this window with an appropriate seal interposed between the housing and insulation (see, for example, U.S. Pat. Nos. 3,939,882 and 4,545,133). Alternatively, again at relatively low pressures, various injection elbows and terminations having the required sealing means and injection port, and developed specifically for these purposes, could be employed (see, for example, U.S. Pat. Nos. 4,888,886; 4,945,653; 4,946,393; 6,332,785; 6,489,554 and 6,517,366).
At higher injection pressures (e.g., 30 to 2000 psig), a greater effort must be made to prevent the liquid from escaping. One connector employed a seal of the FasTest® type which comprises an elastomeric washer co-axially disposed over the insulation jacket and axially compressed between two similar metal washers within a surrounding housing so as to deform the elastomer and thereby form a seal between the insulation and the housing (e.g., see U.S. Pat. Nos. 2,080,271 and 4,345,783). This type of seal was used in the above-cited Transmission & Distribution World injection, wherein a setscrew on one of the metal washers was applied to the crimp connector during injection in order to prevent the connector from popping off the cable conductor due to the higher pressure and to make an electrical connection to the housing.
In some cable injection operations employing relatively high pressures (e.g., new cable injection with a strand blocking compound), Kellems grips (also known as “Chinese fingers”), applied either over the insulation jacket or over the unstripped cable, have been employed in combination with the above-mentioned FasTest® type connector, again to keep the latter from popping off the cable end. Optionally, hose clamps were applied over the Kellems grips to further secure the latter. However, both the injection adaptor and Kellems grips were always removed once injection was completed. (e.g., Bertini et al. Silicone Strand-Fill: A New Material and Process, Spring 1990 Insulated Conductors Committee (ICC) of the Power Engineering Society (PES) of the Institute of Electrical and Electronic Engineers (IEEE), Dearborn, Mich.)